1. Field of the Invention
The present invention relates to a semiconductor isolation structure and fabricating method therefor. Particularly, the present invention relates to a semiconductor isolation structure and a fabricating method in which the size of the active region is kept from decreasing because of an increase in the size of the isolating region, by utilizing a trench.
2. Description of Related Art
As the density of semiconductor devices increases, reducing the size of an isolating region in such semiconductor devices is more and more important.
Generally, a semiconductor device is isolated by applying the LOCOS (local oxidation of silicon) method. In the LOCOS method, a pad oxide layer is formed between a silicon nitride layer and a semiconductor substrate by thermal oxidation in order to relieve stress caused by different rates of thermal expansion between the semiconductor device and the silicon nitride layer. The silicon nitride layer is formed on the active region and is used as a hard mask. Further, a field insulating layer defines a device isolating region which is formed by oxidizing the field region of the semiconductor substrate not covered by the silicon nitride layer. The field insulating layer grows in the vertical direction and in the horizontal direction relative to the substrate because of an oxidant (O2) diffusing along the pad oxide layer. Therefore, the field insulating layer grows under the pattern edge of the silicon nitride layer as well as on the field region.
The encroachment of the field insulating region into the active region is called a xe2x80x9cbird""s beakxe2x80x9d. The length of the bird""s beak eventually becomes one half of the thickness of the field insulating region. Therefore, if the size of the active region is to be kept from decreasing, the bird""s beak must be minimized.
In order to reduce the length of the bird""s beak, a conventional method is known in which the thickness of the field insulating region is decreased. However, if the thickness of the field insulating region is reduced in a 16M DRAM or higher, then parasitic capacitance between the wiring layer and the semiconductor substrate increases. Therefore, the signal transmitting speed is lowered. Further, parasitic transistors are formed in the isolating regions due to the wiring of the gate. Accordingly, the threshold voltage Vt is lowered, with the result that the device isolating characteristics deteriorate.
Therefore, conventional methods are known in which the length of the bird""s beak is decreased, and at the same time, the device isolating characteristics are improved. One of these methods is PBLOCOS (poly Si buffered LOCOS), in which the, thickness of the stress absorbing pad oxide layer is reduced, and a poly crystalline silicon layer is inserted into between the semiconductor substrate and the silicon nitride layer. Another is SILO (sealed interface LOCOS) in which the side walls of the pad oxide layer are protected by a silicon nitride layer. A third one is recessed oxide LOCOS in which a recessed field insulating layer is formed on the field region of the semiconductor substrate.
However, in the above conventional methods, too precise a flatness and too precise a design rule are required to make these methods applicable to the 256M DRAM scale.
Therefore, in order to overcome the above problems, a BOX (buried oxide) type shallow trench isolation method is best known. In this method, a shallow trench is formed on the semiconductor substrate. Thereafter, a buried silicon oxide layer is deposited by a chemical vapor deposition method (CVD method). Therefore, the bird""s beak is not formed, so the active region is not encroached. Further, the silicon oxide layer is buried into the trench and subsequently etched back, thereby obtaining a flat surface.
FIG. 1 is a sectional view showing an isolation structure according to the conventional art.
In this isolation structure, a field insulating layer 23 is formed within a trench 17 defining the isolating region of the semiconductor substrate 11. That is, CVD is used to deposit the field insulating layer 23 into the trench 17. For this purpose, after depositing an insulating layer 23 composed of silicon oxide or the like using CVD, the insulating layer is etched back so that the insulating layer 23 remains only within the trench 17.
That is, the trench 17 is filled with the field insulating layer 23 by applying a CVD process instead of an oxidation process. Therefore the formation of the bird""s beak is prevented. Further, etching back is performed so as to make the silicon, oxide insulating layer 23 remain within the trench. A flat surface is therefore obtained.
FIGS. 2 to 5 illustrate a method for fabricating the above-described conventional isolation structure.
Referring to FIG. 2, a pad oxide layer 13 is formed on semiconductor substrate 11 by a thermal oxidation process. A silicon nitride layer 15 is formed on the pad oxide layer 13 by CVD. A photolithography process is then carried out to form a photolithographic pattern, so that predetermined portions of the pad oxide layer 13 and the silicon nitride layer 15 are removed to thereby expose the substrate 11. This defines a device isolating region I1 and an active region A1. By using the silicon nitride layer 15 as a mask, reactive ion etching (to be called xe2x80x9cRIExe2x80x9d below) is carried out to form the trench 17.
Referring to FIG. 3, silicon oxide is deposited by CVD to fill the trench 17, thereby forming an insulating layer 19. Here, because of the height difference between the surface of the silicon nitride layer 15 and the bottom of the trench 17, a step is formed on the surface of the insulating layer 19.
A photoresist layer 21 is coated on the insulating layer 19. The photoresist layer 21 is patterned to expose locations corresponding to the silicon nitride layer 15 and to remain over locations corresponding to the trench 17 on the insulating layer 19. Then by using the patterned photoresist layer 21 as a mask, the exposed portions of the insulating layer 19 are etched. Here, the etched surface of the insulating layer 19 is roughly as high as the top of the trench 17.
Then, as shown in FIG. 4, the patterned photoresist layer 21 is removed. Then the insulating layer 19 is partially removed to expose the silicon nitride layer 15 by, for example, chemical-mechanical polishing process (to be called xe2x80x9cCMP processxe2x80x9d below).
Then, as shown in FIG. 5, a wet etching process is carried out to etch the pad oxide layer 13 and the silicon nitride layer 15 so as to expose the substrate 11. Under this condition, the upper portion of the insulating layer 19 is also removed so that it remains only within the trench 17. Furthermore, its surface becomes even with the substrate 11, thereby ensuring a flat surface. Thus the insulating layer 19 which remains within the trench 17 becomes a field insulating layer 23.
In this related art, when the pad oxide layer 13 is etched, the upper portion of the filled insulating layer 19 is also etched, and thus a field insulating layer 23 is formed. Under this condition, however, the insulating layer 19 is isotropically etched. Therefore, the field oxide layer is formed to collapse the field insulating layer at the portion which contacts the side wall of the trench. Due to the collapsed portion of the field insulating layer, when a subthreshold voltage is supplied, a hump phenomenon occurs. As a result, current leakage increases, and the refresh characteristics deteriorate. Further, the electric field is concentrated at the top edges of the trench. Therefore, the threshold voltage of the subsequently formed gate insulating layer becomes weak.
The present invention is intended to overcome the above-described disadvantages of the related art.
It is an object of the present invention to provide an isolation structure that avoids increased leakage currents and deterioration in refresh characteristics at a portion of the field insulating layer contacting the side wall of the trench.
It is another object of the present invention to provide an isolation structure that avoids deterioration of the threshold voltage characteristic of the gate insulating layer.
It is still another object of the present invention to provide a fabricating method for an isolation structure that avoids collapse of the portion of the field insulating layer contacting the side wall of the trench.
In achieving the above objects, an isolation structure according to the present invention includes: a semiconductor substrate having a trench defining an isolating region and an active region of the semiconductor substrate; and first, second, and third insulating layers deposited in the trench, the second insulating layer being composed of a material with an etch selection ratio different from those of the first and third insulating layers.
In another aspect of the present invention, the fabricating method for an isolation structure according to the present invention includes: forming a masking layer on a semiconductor substrate to expose an isolated region; forming a trench in this exposed region; depositing a first insulating layer within the trench, leaving a remaining portion of the trench unfilled; removing the masking layer; forming a second insulating layer over exposed side walls of the trench and the surface of the first insulating layer, where the second insulating layer is composed of a material with an etch selection ratio different from that of the first insulating layer; forming a poly crystalline silicon layer upon the second insulating layer to fill the vacant portion of the trench; oxidizing the poly crystalline silicon layer to form a third insulating layer so as to make it project and so as to cover the top edges of the trench; and removing the exposed portions of the second insulating layer.
In still another aspect of the present invention, the fabricating method according to the present invention includes: forming a masking layer on a semiconductor substrate to expose an isolating region; forming a trench in the region thus exposed; forming a first insulating layer within the trench, leaving a remaining portion of the trench unfilled; removing the masking layer; forming a second insulating layer over the exposed side walls of the trench and over the surface of the first insulating layer, where the second insulating layer is composed of a material with an etch selection ratio different from that of the first insulating layer; forming a silicon oxide layer upon the second insulating layer and over the substrate; and etching back the silicon oxide layer to form a third insulating layer, where the third insulating layer fills the remaining vacant portion of the trench, and where the edges of the second insulating layer are exposed.